Light has a characteristic size, the wavelength, which sets a limit to all conventional optical device sizes, especially waveguides and resonators. This limit called the diffraction limit is a fundamental obstacle and is defined as the size of the optical mode in a resonator or waveguide. The value is given by (λ/2ncore)^3 in a 3D geometry, (λ/2ncore)^2 in a 2D geometry and (λ/2ncore)^1 in a 1D geometry. Here, λ/2ncore is the wavelength in free space divided by twice the value of the refractive index of the core of the waveguide or resonator. The ability to overcome this limit is key to future photonic integrated circuits combining nano-scale electrical and micron-scale optical signals. It is also key to future active devices and lasers with a size the fraction of the wavelength. Coupling light to free electrons of metals leads to plasmonic waveguides that can overcome this limitation, but their high optical losses cause a reduced propagation length and high power consumption, a major impediment for integrated photonic circuits.
Modern communication and computation systems rely on the ability to route and transfer information using electronic and electromagnetic signals. Massive efforts over the last decade have been driven by miniaturization and integration of electronics and photonics on the same platform. Relying on optical waveguides as interconnects can increase the speed as well as functionality of integrated circuits, however, the diffraction limit of light is a fundamental barrier to interface micron scale waveguides to nanoscale electronic circuitry. Furthermore, dense photonic integration is hampered because crosstalk between waveguides increases as the separation between them is reduced.
At low frequencies metals, due to their high reflectivity, can be used for confining light at the subwavelength scale. At optical frequencies, metals can achieve the same task by coupling light to free electrons. This leads to a surface plasmon polariton (SPP) which shows properties of nanoscale waveguiding. A number of architectures have recently emerged to effectively utilize the SPP for waveguiding. These include the long range SPP on metal strip (IMI; wherein I≡insulator, M≡metal) waveguides which are useful for sensing applications but not for sub-diffraction confinement. The inverse design, MIM waveguides, confines light to subwavelength scales but leads to a low propagation length. V-groove and wedge plasmon are an excellent candidates for relatively long range propagation and sub-diffraction confinement, however excitation and detection of these modes as well as interfacing with existing silicon waveguide technology are a major challenge.
Recently, hybrid dielectric-plasmonic waveguides have emerged that confine light in a high index gap above metals reducing the field penetration in the metal thus allowing for increased propagation length. Another alternative is an epsilon-near-zero metamaterial waveguide which allows modes to tunnel through subwavelength size structures. However, due to absorption in metals, the above mentioned sub-diffraction plasmonic structures cannot guide light more than a few microns. Furthermore, the dissipated energy leads to thermal issues which are especially significant in miniaturized circuits hindering dense photonic integration.
It is, therefore, desirable to provide light confinement especially in an optical waveguide, and a cladding therefor, that overcomes the shortcomings of the prior art.